FISHES — CYPEINIDAE ACOMUS GENEROSUS. 



221 



The origin of the ventrals is situated in advance of the middle of the dorsal. The tijis of the 

 pectorals reach a vertical line drawn* from the origin of the dorsal. 



D 15 ; A 10 ; C 4, 1, 8, 8, 1, 3 ; V 10 ; P 17. 



There are two rudimentary rays at the anterior margin of both the dorsal and the anal, and 

 one at the exterior margin of the ventrals ; these are all summed up in the formula. Thirteen 

 rows of scales may be counted between the origin of the ventrals and the anterior margin of the 

 dorsal ; the scales themselves are longer than deep, provided with radiating furrows upon their 

 anterior and posterior sections, the margin of which is irregularly convex. The lateral line 

 runs through the median row of scales. 



References to the figures. — Plate XLIX, fig. 1, represents, size of life, Ptychostomus haydeni, 

 from the Yellowstone river. Fig. 2, a scale from the dorsal region. Fig. 3, a scale from the 

 lateral line. Fig. 4, a scale from the abdominal region. 



List of specimens 



Whence obtained. 



Col. A. Vaughan . 

 Gov. I. I.Stevens . 



Nature of 

 specimen. 



Alcoholic 

 ,...do.... 



CoUecteil by — 



Dr.F. V.Hayden 

 Dr. John Evans. 



ACOMUS, Girard. 



Gen. Char. — The head is very elongated ; the lips being papillated and the lower one very deeply cleft. The dorsal fin is higher 

 than long, and the scales are much smaller upon the anterior region of the body than posteriorly. The pharyngeals are 

 gently arched and not expanded ; the teeth, compressed and bituberculated, the inner projection conspicuous; the outer one 

 obsolete though existing. 



SYS.—Acomus, Gkd. in Proc. Acad. Nat. Sc. Philad. VIII, 1856, 173. 



The lips being papillated and the inferior one cleft as in Catostomus, the distinction between 

 this genus and the one just alluded to consists chiefly in a more elongated head and the dispro- 

 portion in the size of the scales of the anterior and posterior region of the body. The 

 pharyngeal bones and teeth will also afi'ord some structural peculiarities, enabling us to 

 discriminate between the species belonging to either of these two genera. 



1. ACOMUS GENEROSUS, Grd. 



Spec. Char. — Head constituting the fifth of the total length; mouth moderate sized; lips rather small, covered with uniform 

 granules. Eye email, sub-circular ; its diameter entering five times in the leugth of the side of the head. Anterior margin of 

 dorsal fin equi-distant between the extremity of the snout and the insertion of the caudal. Extremities of ventrals not reaching 

 the vent; their origin taking place under the posterior third of the dorsal. Olivaceous above, blotched with blackish; yellowish 

 olive beneath. Fins unicolor. 



Syn. — Catostomus (Acomus) generosus, Gri). in Proc. Acad. Nat Sc. Philad. VIII, 1856, 174. 



It is a rather short and contracted species, particularly when compared to A. griseus. The 

 head constitutes about the fifth of the total length. The lips are less developed than in A. 

 griseus, and the tubercles uniform throughout. The eye is moderate sized and sub-circular ; its 

 diameter entering five times in the length of the side of the head. The anterior margin of the 

 dorsal fin is equi-distant between the tip of the snout and the insertion of the caudal fin. The 



